A reed switch is a magnetically controlled switch that automatically makes, breaks, or holds contact in an electrical circuit. Such switches are widely used for electrical circuit control, particularly in the communications field.
The conventional form of dry reed switch includes two magnetic metal reeds which have overlapping, flat, end portions separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope filled with an inert atmosphere. When a magnetic field is applied to such a switch, the flat, overlapping portions of the reeds are attached to each other, thus closing the switch and conducting an electric current. The stiffness of the reeds causes them to separate, and open the circuit, upon removal of the magnetic field. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds.
In production, a metal reed is inserted in each end of a glass tube and the end of the tube thermally softened to collapse about and seal to a shank portion on the reed. This sealing operation has been greatly facilitated by drawing the tubular enclosure from an infrared absorbing glass, and using an infrared heat source to concentrate heat in a small sealing zone on the glass tube.
The procedures and materials heretofore used in infrared sealing practices have proven quite satisfactory for many purposes. Recently, however, difficulties were encountered in attempts to develop a miniaturized reed switch. The proximity of the glass-metal seal areas to the gold plated contact surfaces of the reeds, and the high temperature necessary for sealing, caused the gold coating on the reeds to blister. Thus, it became highly desirable to provide a glass envelope that could be sealed at a lower temperature while still retaining the other necessary characteristics of a reed switch glass, namely high electrical resistivity, resistance to devitrification and a reasonably high degree of chemical durability. Recent emphasis on avoiding air pollution made it further desirable to produce the envelope from a glass which omitted the well-known, volatile glassmaking materials, lead oxide and fluorides, from its composition.